People could eat like pigs and still avoid obesity with active lifestyles; the real problem is that too many people today go from their bed to a desk at work to their couch at home (and the cycle repeats itself).
It would still depend a lot on what they eat. Two hours of physical work can be ruined by 45 seconds with a bag of cookies. The fat people I see on the treadmills are “exercising” for a lot longer than me, probably by a 5 to 1 ratio or better over the course of a week. Something is making them stay fat.